1. Field of the Invention
The present invention relates to a low-voltage joystick port interface and a method of interfacing a standard-voltage joystick with a low-voltage port of a processor.
2. Description of Prior Art
As a peripheral device, a user manipulated joystick enables the real-time interaction between a user and a host computer which is necessary for certain computer applications (e.g., computer games). The joystick typically includes a potentiometer for each orthogonal coordinate axis. The resistance of the potentiometer varies in direct relation to the joystick handle position along the corresponding coordinate axis. Each potentiometer has a first terminal connected to a 5 Volt supply. To provide digital values which may be processed by the host computer, a second terminal of the joystick potentiometer is connected to a joystick port interface.
As illustrated in FIG. 1, the prior art joystick port interface 120 (illustrated for a single coordinate axis only, e.g., the X-axis) includes a quad timer 126 and a "recommended" Resistor-Capacitor (RC) network having a resistor 122 (typically R=2.26 kilohms) and a capacitor 124 (typically C=10 nF). A first terminal of the RC network resistor 122 is serially coupled to the joystick potentiometer 112, while the other terminal of the RC network resistor 122 is coupled to a node A. A first terminal of the RC network capacitor 124 is coupled to the node A, while the other terminal of the RC network capacitor 124 is connected to ground. The quad timer 126 is coupled to the node A, and receives the analog voltage level, JSout, across the RC network capacitor 124. The quad timer 126 includes an analog comparator (not shown) which compares JSout with a predetermined threshold voltage Vt (typically 3.34 Volts) and outputs a pulse signal P.sub.i to the host computer.
Upon receiving a request from the host computer, the quad timer 126 discharges the RC network capacitor 124 and sets P.sub.i to a logic "1" level. As current passes though the joystick potentiometer 112, the RC network capacitor 124 charges until Vt is reached. At this time the quad timer 126 sets P.sub.i back to a logic "0" level. The pulse width of P.sub.i thus represents the time interval, T, required to charge the RC network capacitor 124 to the threshold voltage Vt. The pulse width of P.sub.i is monitored by the host computer to indicate the resistance of the joystick potentiometer 112 which, as discussed above, has a direct relation to the coordinate position of the joystick 110.
For the conventional joystick port interface described above, both the joystick 110 and the quad timer 126 utilize a 5 Volt power supply. The power supply for the next generation of integrated circuits, however, will be substantially less than 5 Volts, and therefore a low-power port is needed to interface the conventional 5 Volt joystick device with a lower-Volt integrated circuit such as a CMOS (complementary metal-oxide silicon) VLSI (very large-scale integration) circuit.